No billing in remote function list

I want integrate ispconfig with my page by remote api. But I don't know why if I list available functions by function get_function_list - I don't see any of billing function. Billing module work properly since 2 years and remote user has checked Billing functions.

Where I should search error?

Another question - not really important: when I can change default VAT value? At this moment any new
article have 19% VAT. Is this in database stored or I must search and replace in billing files?